With remarkable advances in the performance of computers and other information equipment, the volume of information that is handled by users has been constantly increasing and the unit of measurement is now in terabytes rather than gigabytes. Under these circumstances, there exists an ever-growing demand for semiconductor devices such as information storage/reproduce equipment and memories that are capable of recording at even higher densities.
To achieve higher recording densities, technologies for even finer microfabrication are required. Conventional photolithography which uses the exposure process is capable of microfabrication over a large area in one step; however, since its resolution is not finer than the wavelength of light, conventional photolithography is inevitably unsuitable for creating fine structures smaller than the wavelength of light (say, 100 nm and less). Technologies currently available for processing finer structures than the wavelength of light include exposure using electron beams, exposure using X-rays, and exposure using ion beams. However, pattern formation with an electron beam lithographic apparatus differs from patterning by one-shot exposure using such light sources as i-line and an excimer laser in that the more patterns that need be written with electron beams, the longer the time that is required for writing (exposure). Therefore, as the recording density increases, the time it takes to form a fine-featured pattern is prolonged to cause a marked drop in throughput. With a view to forming patterns at a faster speed by the e-beam lithographic equipment, the development of a method for one-shot irradiation of geometric figures is underway in which combinations of variously shaped masks are subjected to one-shot exposure to electron beams; however, the e-beam lithographic apparatus that uses the method for one-shot irradiation of geometric figures is not only bulky but it also needs an additional mechanism for controlling the positions of masks to an even higher precision; this increases the cost of the lithographic apparatus, eventually leading to a higher cost for manufacturing the media.
Printing-based approaches have been proposed as an alternative to the conventional exposure technologies for creating fine structures smaller than the wavelength of light. See, for example, US 005772905A which describes an invention relating to the technology of nanoimprint lithography (NIL). The technology of nanoimprint lithography (NIL) is a technique in which a pattern of predetermined fine structures is formed on a mold by exposure to electron beams or using some other methods of creating finer structures than the wavelength of light and the mold is urged under pressure against a resist-coated transfer substrate so that the fine-structured pattern is transferred to the resist coating on the transfer substrate. As long as the mold is available, there is no particular need to employ an expensive exposure unit but an apparatus in the class of ordinary printing presses will suffice to produce replicas in large quantities; hence, in comparison with the conventional methods such as exposure to electron beams, there is achieved a marked improvement in throughput whereas the manufacturing cost is significantly reduced.
As descried in US 005772905A, when a thermoplastic resin (say, PMMA) is used as a resist material in the technology of nanoimprint lithography (NIL), transfer is performed with the thermoplastic resin being heated under pressure to a temperature near its glass transition temperature (Tg) or higher. This approach is called a thermal transfer process. The thermal transfer process has the advantage of permitting the use of general-purpose, thermoplastic resins. If a photosensitive resin is used as a resist in the NIL technology, a photocurable resin that hardens upon exposure to light such as UV radiation is chosen as the resin to which the original fine-featured pattern is transferred. This approach is called an optical transfer process.
In the nanoimprint processing technology using the optical transfer process, a special photocurable resin must be used but, on the other hand, compared to the thermal transfer process, the optical transfer process has the advantage of reducing the dimensional errors in finished products due to the thermal expansion of transfer printing plates or printing media. Other advantages that are related to the apparatus include elimination of the need for equipping it with a heating mechanism and providing accessories such as for performing temperature elevation, temperature control, and cooling. There is a further advantage concerning the nanoimprint apparatus taken as a whole and that is elimination of the need for design considerations against thermal distortions, such as heat insulation.
An example of nanoimprint apparatuses based on the optical transfer process is described in JP 2008-12844A. This apparatus is so designed that a stamper capable of transmitting UV light is urged against a photocurable resin coated transfer substrate and irradiated with UV light from above. The stamper has a predetermined pattern of fine structures formed in the surface that is to be pressed against the transfer substrate.
FIG. 9 in the accompanying drawings is a schematic diagram showing major steps in a fine-structure transfer method involving the nanoimprint technology based on the optical transfer process. In step (a), a transfer element 100 comprising a substrate 102 coated with a resist 104 on its topside is placed in a face-to-face relationship with a stamper 108 having a fine-featured pattern 106 formed on the side that is to be brought into contact with the resist 104. In step (b), the stamper 108 is pressed against the resist-coated surface of the transfer element 100. In step (c), ultraviolet (UV) light is applied to the stamper 108 from above, whereby the resist 104 is hardened. Then, in step (d), the stamper 108 is detached from the transfer element 100, leaving a patterned layer 110 on a surface of the substrate 102 of the transfer element 100. The patterned layer 110 is the obverse image of the fine-featured pattern 106.
To press the stamper 108 against the transfer element 100, a press mechanism is used, as indicated by 111 in FIG. 10. The conventional press mechanism 111 generally comprises a uniaxial moving stage 112 which is a combination of a linear guide and a servo motor. With the transfer element 100 being fixed on a base 114, the stamper 108 mounted on the moving stage 112 is moved down so that it is urged against the transfer element 100. The moving stage 112 is controlled by an XYZ control circuit 116. Since the positions of the transfer element 100 and the stamper 108 are important for precision in transfer, the moving stage 112 requires precision in both motion and position. Consequently, the press mechanism 111 has assumed a bulky and complicated structure.
Another problem with the conventional press mechanism 111 is the occurrence of transfer defects due to air bubbles. If the pressing of the stamper 108 in the position shown in step (a) of FIG. 9 into intimate contact with the resist 104 as shown in step (b) is performed in the atmosphere, air bubbles are trapped in the space between the recesses in the fine-featured pattern 106 on the stamper 108 and the resist 104 and even after the pressing action is over, those air bubbles will stay unremoved, causing the external shape of the resist 104 to be fixed as irregularly deformed. This makes it difficult or even impossible to ensure that the shape of the fine-featured pattern 106 is transferred correctly.
Still another problem with the conventional press mechanism 111 is the difficulty involved in detaching the stamper 108 from the transfer element 100. After the stamper 108 has been pressed into contact with the transfer element 100 and pattern transfer effected as shown in step (c) of FIG. 9, the stamper 108 has to be detached from the transfer element 100 in subsequent step (d). If the stamper 108 and the transfer element 100 are securely adhered to each other via the hardened resist 104, it is extremely difficult to detach the stamper 108 from the transfer element 100. A wedge or the like may be inserted from the outside to form a gap between the transfer element 100 and the stamper 108 so that the stamper 108 can be gradually detached from the transfer element 100; however, this method takes so much time and labor that the operating efficiency drops considerably. In addition, when a wedge or the like is being driven into the interface between the transfer element 100 and the stamper 108 that are in intimate contact with each other, the transfer element 100 and/or the stamper 108 might be damaged.